This invention has as an object a process for producing automotive post-combustion catalysts on metal or ceramic substrates or other refractory inorganic materials. It also has as its object the catalysts obtained by this process.
These catalysts can be used, for example, in the treatment of exhaust gases from internal combustion engines with controlled or non-controlled ignition, operating with a liquid fuel, a liquified petroleum gas (LPG), or a compressed natural gas (CNG).
For monolithic substrates having a beehive (or honeycomb) structure or obtained by stacking of metal strips or else produced by the interlocking of fibers, either metal or ceramic, to be able to receive a well-dispersed, catalytically active phase (generally precious metals) (the catalytic activity generally being proportional to the exposed metal surface area), it is important that the specific surface area of such substrates (which is almost the same as the geometric surface area and is equal to several m.sup.2 per liter of substrate) be substantially increased. Covering the surface of the substrate with a refractory inorganic oxide having a large surface area (100-200 m.sup.2 /g) is one way to accomplish this.
Other criteria also have to be met in order for the catalyst to perform well: actually, to keep the performance of the catalyst from being excessively diminished by rapid and intermittent changes in operating conditions, or to impart a certain stability to the catalyst (thermal stability of the substrate and/or of the catalytic phase, reduced sensitivity to potential poisons), it is desirable to add to this inorganic oxide of large surface area compounds that are able to attenuate the extent of these detrimental effects.
It is well known that cerium and iron oxides (particularly U.S. Pat. No. 4,378,307 and U.S. Pat. No. 4,294,726) or other oxides such as those of rare earths or of alkaline-earth metals or else of zirconium (U.S. Pat. No. 4,587,231 or U.S. Pat. No. 4,791,091 or FR-A-2568143) can provide such improvements. In general, these agents are introduced either in the form of pigments in the coating suspension or by impregnating precursor salts on the refractory oxide before being put into suspension, or by impregnating the coated monolith.